ui/phase-19: ship-group decoder + map binding + selection store

Wires Phase 19's data and rendering layers without yet adding the
inspector UI:

  - game-state.ts grows ReportLocalShipGroup / ReportOtherShipGroup
    / ReportIncomingShipGroup / ReportUnidentifiedShipGroup /
    ReportLocalFleet types and walks the matching FlatBuffers
    vectors (LocalGroup, OtherGroup, IncomingGroup,
    UnidentifiedGroup, LocalFleet) inside decodeReport. The Tech
    map is folded into the fixed-shape ShipGroupTech struct;
    cargo strings normalise to the closed CargoLoadType | "NONE"
    union; UUIDs come back as canonical 36-char strings.
  - synthetic-report.ts mirrors the new fields so the DEV-only
    lobby loader can feed JSON produced by legacy-report-to-json
    straight into the live UI surface.
  - selection.svelte.ts widens its discriminated union with a
    `kind: "shipGroup"` branch carrying a ShipGroupRef
    (local UUID / other / incoming / unidentified by index).
  - world.ts adds Style.strokeDashPx and render.ts.drawLine
    honours it via manual segmentation (PixiJS v8 has no native
    dash API). Ignored on points and circles.
  - state-binding.ts now returns { world, hitLookup }: the
    hit-lookup map keys every primitive id back to a concrete
    HitTarget so the click handler can dispatch to selectPlanet
    or selectShipGroup. Ship-group primitives live in a separate
    ship-groups.ts that emits one point per local / other /
    unidentified group, plus a dashed origin→destination line +
    clickable point per incoming group. Position is interpolated
    along the trajectory for in-hyperspace groups.
  - map.svelte threads the hitLookup into handleMapClick.

Vitest:
  - tests/helpers/empty-ship-groups.ts exposes EMPTY_SHIP_GROUPS
    so existing fixtures can spread the new five empty arrays
    without enumerating every field.
  - state-binding-groups.test.ts covers each group variant's
    primitive geometry and lookup correctness.
  - All previously-existing fixture builders pick up the spread
    so GameReport stays a complete object.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

ui/frontend/src/lib/selection.svelte.ts

@@ -1,7 +1,7 @@
 // Per-game selection state: which on-map object the user is
-// currently inspecting. Phase 13 only models planet selection, so
-// the union has a single variant; later phases (Phase 19 ship-group
-// inspector) will widen it.
+// currently inspecting. Phase 13 modelled planets only; Phase 19
+// widened the union to ship groups (own / foreign / incoming /
+// unidentified).
 //
 // The store is in-memory only: lifetime matches the in-game shell
 // layout instance, which itself is preserved across active-view
@@ -20,12 +20,30 @@
 // can be tested directly without rendering any UI.
 
 /**
- * Selected describes the currently selected map object. Phase 13
- * ships only the planet variant; later inspector phases extend the
- * discriminated union (`ship-group`, etc.) without changing the
- * store's contract.
+ * ShipGroupRef identifies a ship group inside the current report.
+ * `local` groups carry a stable engine UUID (passed through
+ * `report.localGroup.id` and used by the upcoming Phase 20 order
+ * envelopes). The remaining variants do not — they are addressed by
+ * their position in the matching report array, which is fine for
+ * the read-only inspector: a new report load reseeds the store and
+ * any stale index resolves to a missing entry on lookup, collapsing
+ * the inspector cleanly.
  */
-export type Selected = { kind: "planet"; id: number };
+export type ShipGroupRef =
+	| { variant: "local"; id: string }
+	| { variant: "other"; index: number }
+	| { variant: "incoming"; index: number }
+	| { variant: "unidentified"; index: number };
+
+/**
+ * Selected describes the currently selected map object. The
+ * discriminated union is closed: every map-clickable surface maps
+ * to one of these variants. Future phases (e.g. fleet selection)
+ * extend by adding a new branch — extension is purely additive.
+ */
+export type Selected =
+	| { kind: "planet"; id: number }
+	| { kind: "shipGroup"; ref: ShipGroupRef };
 
 /**
  * SELECTION_CONTEXT_KEY is the Svelte context key the in-game shell
@@ -49,6 +67,16 @@ export class SelectionStore {
 		this.selected = { kind: "planet", id };
 	}
 
+	/**
+	 * selectShipGroup sets the active selection to a ship group. The
+	 * `ref` discriminator carries the variant + the right id shape for
+	 * lookup against the current report.
+	 */
+	selectShipGroup(ref: ShipGroupRef): void {
+		if (this.destroyed) return;
+		this.selected = { kind: "shipGroup", ref };
+	}
+
 	/**
 	 * clear drops the current selection. The mobile sheet's close
 	 * button calls this; otherwise selection persists across active-